1. Field of the Invention
This invention relates to the reduction of pollutants produced by gas-fired combustion systems, and more particularly to a method amd apparatus for inhibiting the formation of oxides of nitrogen (NO.sub.x) in gas-fired forced air furnaces.
2. Background and Related Art
The combustion process for gas-fired furnaces generates gaseous combustion products, including NO.sub.x, which are vented to the atmosphere as flue gases. Since NO.sub.x is a pollutant, it is desirable to limit the emission of NO.sub.x into the atmosphere and toward this end some jurisdictions require that combustion systems meet strict NO.sub.x emission standards. Several techniques have been used to inhibit the formation of pollutants in combustion systems. For example, U.S. Pat. No. 3,726,633, issued Apr. 10, 1973 to A.D. Vasilakis et al, teaches that pollutants from a liquid-fired burner can be reduced by more completely combusting the fuel. In the system of the Patent, air and fuel are supplied to a burner and the flame is directed into a wire mesh cage formed from a cylindrical radiant wire mesh screen bounded at the ends by ceramic plates, one of which has an aperture through which the flame is introduced into the cage. The other end plate of the cylinder is a plain disc of castable firebrick. The screen is described as being made of Inconel, Nichrome 5, silicon carbide fibers or other high-temperature oxidation resistant material (see column 1, lines 61-65). The cylindrical screen is heated by radiation from the combustion flame and by convection from the combustion gases falling through the screen. The screen loses heat to the ambient volume principally by radiation and maintains an equilibrium temperature lower than that at which NO.sub.x is formed. Complete combustion with minimum excess air and low pressure drop is stated to be achieved, resulting in operation at high thermal efficiency with minimal pollutant production.
U.S. Pat. No. 4,776,320, issued Oct. 11, 1988 to Ripka et al, is directed to a device for use in a combustion system to inhibit formation of oxides of nitrogen. The device is stated to comprise a piece of material positioned relative to the combustion flame produced by a burner to temper the combustion by absorbing thermal energy from the combustion flame. The device, called a flame radiator structure, is a perforated tubular structure having a generally rectangular cross section. The device is shown in FIG. 2, noted number 11. Also, an alternate design for the device is stated to be a stainless steel cylindrical screen shown in FIG. 5. It is made of stainless steel material 30 having mounting flanges 31 as shown in FIG. 5. It is stated that a desirable location for the flame radiator structure relative to the combustion flame produced by a two-zone combustion-type burner is at the periphery of the combustion flame as shown in FIG. 4 where the structure 11 surrounds secondary combustion zone 34 to efficiently and effectively reduce peak flame temperatures and residence time at the peak flame temperatures to desired levels (see column 4, lines 35-60). In single-zone combustion systems the structure 11 may be disposed within the combustion zone (see column 5, lines 18-29). However, the flame radiator structure should not bend into the combustor flame (see column 7, lines 10-30).
U.S. Pat. No. 2,655,944, to Proehl, dated Oct. 20, 1953, discloses a heating tube for use in a deep fat fryer. As shown in FIGS. 1 and 5, the heating tube may comprise a wire mesh dividing baffle 10. Baffle 10 may be made of Nichrome and it is disposed between burners, parallel to the direction of flow of combustion products in the conduit, so that the burners inject flames into spaces above and below, but not through, the baffle. The baffle is employed to radiate heat from the burners to the sides of the tube (see column 2, lines 25-42). Additional baffles may be disposed downstream of baffle 10 transversely to the direction of flow of combustion gases through the tube, as shown in FIGS. 4 and 5.
U.S. Pat. No. 3,187,798 to Pokorny, dated Jun. 8, 1965, discloses a radiant gas burner comprising a conical refractory shell 30 within which a combustion mixture of fuel and air is burned to heat the shell to incandescence. The shell 30 comprises a plurality of holes 40 through which gaseous combustion products escape.
U.S. Pat. No. 4,044,796, issued Aug. 30, 1977 to Smick, describes a flue gas turbulator which is capable of mixing and directing a heating fluid, such as a flue gas, against the internal walls of a heat exchanger conduit such as a fire tube or the like. The turbulator is stated to be formed of a strip of metal bent into a series of alternating deflection panels successively joined together by bridging sections. The deflection panels along the length of the strip are alternately angling back and forth, thus simultaneously tilting up and down relative to the turbulator axis. The turbulator is particularly shown in the Figures.
U.S. Pat. No. 1,632,888 issued Jun. 21, 1927 to Davis et al is directed to a water heater. As shown in FIG. 1 the water heater comprises a tubular flue 13 through which hot combustion products from burner 14 are passed. The flue is thus heated and in turn heats water in the surrounding storage vessel. Tubular flue 13 contains a heat radiating member 21 that has formed from a strip of suitable sheet material bent into zig-zag form with flat portions directed in various directions. It is stated that the flat portions of the heat radiating member tend to deflect and throw the heat from the burner outward in opposite directions, thus rapidly heating the flue.